All closed refrigeration systems serially include a compressor, a condenser, an expansion device and an evaporator. Expansion devices include fixed orifices, capillaries, thermal and electronic expansion valves, turbines, and expander-compressors or expressors. In each of the expansion devices, high pressure liquid refrigerant is flashed as it goes through a pressure drop with at least some of the liquid refrigerant becoming a vapor causing an increase in specific volume. In an expressor, the volumetric increase is used to power a companion compressor which delivers high pressure refrigerant vapor to the discharge of the system compressor thereby increasing system capacity. Since the compression process occurring in the expressor is not powered by an electric motor, but by the flashing liquid refrigerant, overall refrigeration efficiency increases by the same amount as the system capacity.
Screw compressors and expanders are fundamentally unbalanced both axially and radially. Three-port screw expressors with a single low pressure port, as exemplified by commonly assigned U.S. Pat. No. 6,185,956, are still radially unbalanced.
An oil free screw expander-compressor, or expressor, unit is used for phase changing air conditioning and refrigeration systems. The expander functions as a set of timing gears in controlling the relative angular positions of the male and female rotors and driving the companion compressor of the expresser. This is possible since the expander has a liquid refrigerant component of at least 70% which forms a strong dynamic liquid film to separate the male and female rotors. The refrigerant-lubricated expander rotors become a pair of timing gears just like conventional timing gears in a dry screw compressor. The male and female rotors of the compressor portion of the expressor are given a greater clearance and therefore do not contact each other. This characteristic allows oil-free, dry compressor operation for the compressor portion of the expressor, just like a timing gear allows oil-free operation of conventional compressors. The difference between the timing gears of conventional dry compressors and the two phase flow screw expander in the expressor is that the former is a conventional gear transferring torque from a mechanical drive while the latter is itself an expander. The rotors of the expander and compressor of the expressor are oil-free with the expander rotors being lubricated by the liquid portion of the two-phase working fluid, and a dynamic liquid film separates the male and female rotors of the expander.
It is an object of this invention to balance radial and axial gas forces in an expressor.
It is an additional object of this invention to limit rotor distortion thereby allowing reduction of the clearance between the expressor rotors.
It is another object of this invention to reduce bearing loading in an expresser.
It is a further object of this invention to improve expressor performance.
It is an additional object of this invention to use the rotors of the expander as timing gears relative to the rotors of the compressor of the expressor. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.
Basically, the expansion device in a refrigeration or air conditioning system is an expressor. The expressor is made up of a twin screw expander and a twin screw compressor with rotors of the expander functioning as timing gears.